Charge mode active pixel sensor read-out circuit

ABSTRACT

An apparatus such as an imager includes groups of sensors each of which includes subgroups of sensors. Subgroup select circuits are coupled to outputs from respective subgroups of sensors, and group select circuits are coupled to outputs from subgroup select circuits associated with respective ones of the groups. A bus is coupled to receive outputs from the group select circuits. A controller can provide control signals to the subgroup select circuits and the group select circuits to selectively enable the respective subgroup select circuits and group select circuits to pass signals from the sensors to the bus one sensor at a time.

BACKGROUND

The present invention relates to a charge mode active pixel sensor read-out circuit.

Image sensors can be applied in a wide variety of fields, including machine vision, robotics, guidance and navigation, automotive applications, and consumer products. They can include on-chip circuitry that controls image sensor operation, signal read-out and image processing functions. Image sensors can utilize, for example, active pixel sensor (APS) technology with each active pixel sensor including one or more active transistors. Each pixel sensor element can provide an output value that represents a particular portion of an image. The pixel sensor elements can be arranged as an array. Each element in the array may be read sequentially, one element at a time, by a charge mode read-out circuit.

Parasitic capacitance on a readout bus can detrimentally affect APS charge mode read-out circuit operation. Such detrimental effects may include, for example, increased power requirements, increased noise levels and increased signal settling times with consequential slower system operating speed.

SUMMARY

In general, according to one aspect, an apparatus such as an imager includes groups of sensors each of which includes subgroups of sensors. Subgroup select circuits are coupled to outputs from respective subgroups of sensors, and group select circuits are coupled to outputs from subgroup select circuits associated with respective ones of the groups. A bus is coupled to receive outputs from the group select circuits. A controller can provide control signals to the subgroup select circuits and the group select circuits to selectively enable the respective subgroup select circuits and group select circuits to pass signals from the sensors to the bus one sensor at a time.

According to another aspect, a method includes selectively enabling a group select circuit to electrically couple a charge mode read-out amplifier to a respective set of subgroup select circuits, when the group select circuit is enabled, enabling a pixel output signal to pass sequentially from each subgroup select circuit of the respective set of subgroup select circuits through the group select circuit to the charge mode read-out amplifier, and disabling the group select circuit to electrically isolate the charge mode read-out amplifier from the respective set of subgroup select circuits.

According to yet another aspect, a method includes selectively enabling a supergroup select circuit from a set of supergroup select circuits and a series-connected group select circuit from an associated set of group select circuits to electrically couple a charge mode read-out amplifier to a respective set of subgroup select circuits, when the series-connected group select circuits are so enabled, enabling a pixel output signal to pass sequentially from each subgroup select circuit of the respective set of subgroup select circuits through the series-connected group select circuit and supergroup select circuit to the charge mode read-out amplifier, and disabling the group select circuit to electrically isolate the charge mode read-out amplifier from the respective set of subgroup select circuits.

In some implementations, one or more of the following advantages may be present. The techniques described herein can help reduce parasitic capacitance that is electrically coupled to the input ports of a charge mode read-out amplifier. That can reduce losses in a system and reduce the power required for charge mode amplifier operation. This reduction of power requirements may result in operational cost savings for a system.

A reduction in parasitic capacitance can reduce of the magnitude of noise in signals received at the input terminals of the charge mode read-out amplifier. A reduction in noise may result in improved signal clarity and an improvement in the reliability of interpreting such signals. As parasitic capacitance is reduced, signal settling times and system operating speeds also can be improved.

Additionally, the techniques described herein can be implemented without adversely impacting fixed pattern noise (FPN) in a system. In charge mode systems a charge is typically sent through a readout bus to a charge mode amplifier where it is converted into a voltage signal. The techniques described below can be implemented without adversely affecting the output voltage of the charge mode amplifier.

Other features and advantages will be readily apparent from the following detailed description, the accompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram.

FIG. 2 is a schematic diagram.

FIG. 3 is a timing diagram.

FIG. 4 is a schematic diagram.

FIG. 5 is a schematic diagram.

DETAILED DESCRIPTION

As shown in FIG. 1, a monolithic integrated imaging circuit 100 includes an array 102 of active pixel sensors and a controller 104. The controller 104 provides timing and control signals that facilitate reading out signals from the pixel sensors and that facilitate various other image processing functions. The dimensions of the array 102 can vary depending on specific imaging requirements for a particular application. For example, array 102 might have dimensions such as 128 active pixel sensors by 128 active pixel sensors or more.

In the illustrated implementation, the various circuit components are coupled to each other by channels 128. Channels 128 can include one or more conductive paths that carry signals between circuit components. A read-out circuit 116 is electrically coupled to the array 102 through a subgroup select module 108 and a group select module 106. The read-out circuit 116 can be a charge mode amplifier or other circuit responsive to a sensed charge. An active pixel sensor of array 102 provides an output signal representative of a portion of an image, and the read-out circuit 116 receives the output signal. The subgroup select module 108, group select module 106 and read-out circuit 108 may include, for example, sample and hold circuitry, switching circuitry, and signal conditioning circuitry. The read-out circuit provides an output signal in response to the signal received from an active pixel sensor of array 102. The output may take the form, for example, of a differential signal on lines 124 and 126, or a non-differential signal outputted to one or more lines.

The imaging circuit 100 also includes counter circuits 110, 120, decoder circuits 112, 118 and other circuitry to support imaging circuit 100 operations. Such circuitry may include for example, row select circuit module 114, latches 122 and additional levels of select circuitry (not shown).

As shown in FIG. 2, the array 102 includes subgroups 204 a, 204 b . . . 204 y of active pixel sensors 200, that are organized into groups 202 a, 202 b . . . 202 g. Each group 202 a, 202 b . . . 202 g includes one or more subgroups 204 a, 204 b . . . 204 y, for example, group 202 a includes subgroups 204 a, 204 b . . . 204 f. The subgroups 204 a, 204 b . . . 204 y are arranged as columns. Alternatively, the subgroups 204 a, 204 b . . . 204 y can be arranged as rows or in other configurations. In the illustrated implementation, each subgroup 204 a, 204 b . . . 204 y includes two output ports, each of which is electrically coupled by two output lines to an associated subgroup select circuit 206 a, 206 b . . . 206 y. Alternatively, each subgroup 204 a, 204 b . . . 204 y may include only a single output port electrically coupled to an associated subgroup select circuit 206 a, 206 b . . . 206 y through one or more electrically conductive lines.

The subgroup select circuits 206 a, 206 b . . . 206 y are represented as NMOS switches, however, the subgroup select circuits 206 a, 206 b . . . 206 y may include other circuitry such as, sample and hold circuitry, or other signal processing circuitry. Additionally, such circuits could be implemented using NMOS, PMOS or CMOS technologies.

Each subgroup select circuit 206 a, 206 b . . . 206 y includes output ports that are electrically coupled to a differential output bus 208 a, 208 b . . . 208 g. Each differential output bus 208 a, 208 b . . . 208 g may include two or more conductive paths that electrically couple the output ports of all of the subgroup select circuits 206 a, 206 b . . . 206 y associated with a particular group 202 a, 202 b . . . 202 g to an input port of a respective group select circuit 210 a, 210 b . . . 210 g. The group select circuits 210 a, 210 b . . . 210 g can be implemented using NMOS switches, although, other implementations may include PMOS or CMOS switches. Each group select circuit 210 a, 210 b . . . 210 g includes output ports that can be electrically coupled to a common output bus 212. This common output bus 212 can be electrically coupled, for example, to input ports of a readout circuit 116, such as a charge mode read-out amplifier or other signal processing circuit.

The differential output buses 208 a, 208 b . . . 208 g are coupled to a set of clamping voltage switches 216. These switches close when the respective differential output bus 208 a, 208 b . . . 208 g is electrically isolated from the read-out circuit 116, to maintain the bus voltage at a predetermined clamp voltage value. The clamp voltage can be, for example, the power supply voltage or a grounded connection. This can minimize signal distortion and undesirable leakage current in the system.

The controller 104 sends subgroup select signals C_(SS) to the gate terminals of the subgroup select circuits 206 a, 206 b . . . 206 y to enable each subgroup select circuit 206 a, 206 b . . . 206 y to pass a signal from an active pixel sensor 200 of a respective subgroup 204 a, 204 b . . . 204 y to a respective differential bus 208 a, 208 b . . . 208 g. The controller 104 also sends group select signals G_(SS) to the gate terminals of the group select circuits 210 a, 210 b . . . 210 g to enable each group select circuit 210 a, 210 b . . . 210 g to pass a signal from a respective subgroup select circuit 206 a, 206 b . . . 206 y to the common output bus 212.

FIG. 3 is a control signal timing diagram 320 detailing the operation of readout circuit 116. Group select signals G_(SS) for each respective group select circuit 210 a, 210 b . . . 210 g and subgroup select signals C_(SS) for each respective subgroup select circuit 206 a, 206 b . . . 206 y are shown as a function of time along a time axis 304. The group select signals G_(SS) and the subgroup select signals C_(SS) can be, for example, voltage signals generated by the controller 104 or by another circuit or combination of circuits and power sources. The group select signals G_(SS) are provided at gate terminals of each group select circuit 210 a, 210 b . . . 210 g to selectively enable each group select circuits 210 a, 210 b . . . 210 g to pass a signal between the respective drain and source terminals one at a time in a sequential manner. The subgroup select signals C_(SS) are provided to gate terminals of each subgroup select circuit 206 a, 206 b . . . 206 y to selectively enable each subgroup select circuit 206 a, 206 b . . . 206 y to pass a signal between the respective drain and source terminals one at a time, in a sequential manner.

At time 306, a group select signal G_(SS) is applied to the gate terminal of group select circuit 210 a establishing a conductive path between the drain and source terminals of group select circuit 210 a, thereby effectively “closing” the switch. At the same time 306, a subgroup select signal C_(SS) is applied to the gate terminals of subgroup select circuit 206 a establishing a conductive path between the drain and source terminals of the subgroup select circuit 206 a, thereby effectively “closing” the switch. Applying a subgroup select signal C_(SS) to the gate terminals of subgroup select circuit 206 a thus enables the subgroup select circuit 206 a to pass a signal to the group select circuit 210 a. Similarly, applying a group select signal G_(SS) to the gate terminals of group select circuit 210 a enables the group select circuit 210 a to pass the signal to the common bus 212. During this time, the readout circuit 116 is isolated electrically from subgroup select circuits 206 b . . . 206 g that are associated with groups 202 b . . . 202 g.

Immediately following time 306, the charge mode read-out circuit 116 can receive a signal from an active pixel sensor 200 in subgroup 204 a of group 202 a. The signal is passed from subgroup 204 a, to subgroup select circuit 206 a, to differential output bus 208 a, through group select circuit 210 a, to common differential output bus 212, and then to readout circuit 116.

At time 308, the group select signal G_(SS) previously applied to group select circuit 210 a is maintained, but the subgroup select signal C_(SS) previously applied to subgroup select circuit 206 a is changed to disable subgroup select circuit 206 a, effectively “opening” the switch. That has the effect of electrically isolating subgroup 204 a from the readout circuit 116 and electrically coupling subgroup 204 b to the readout circuit 116. A subgroup select signal C_(SS) is then applied to enable subgroup select circuit 206 b.

Immediately following time 308, a signal generated in a pixel of subgroup 204 b can be passed from subgroup 204 b, to subgroup select circuit 206 b, to differential output bus 208 a, through group select circuit 210 a, to common differential output bus 212, and to readout circuit 116.

The subgroup select signals C_(SS) sequentially enable, one at a time, the subgroup select circuits 206 a, 206 b . . . 206 f that are coupled to the group select circuit 210 a. During that time, a group select signal G_(SS) is applied at the gate terminals of group select circuit 210 a to maintain group select circuit 210 a in a conductive state.

At time 310, the group select signal G_(SS) that was previously applied to group select circuit 210 a is removed and a new group select signal G_(SS) is applied to group select circuit 210 b. The subgroup select signals C_(SS) then are sent, sequentially, to each subgroup select circuit 206 g, 206 h . . . 206 l coupled to group select circuit 210 b. While the group select circuit 210 b is in a conductive state, the readout circuit 116 is isolated electrically from the subgroup select circuits not associated with group 202 b.

The read-out circuit 116 continues in this manner to read-out signals sequentially from the subgroup select circuits 206 a, 206 b . . . 206 y until they all have been read and passed to readout circuit 116. The particular order of polling may vary.

FIG. 4 illustrates how the techniques described herein can reduce parasitic capacitance present at the input terminals of an imager's readout circuit 116. An array 102 includes nine subgroups 204 a, 204 b . . . 204 i of active pixel sensors 200. The subgroups 204 a, 204 b . . . 204 i are divided into three groups 202 a, 202 b, and 202 c. Each group 202 a, 202 b, and 202 c includes three subgroups. For example, group 202 a includes subgroups 204 a, 204 b, and 204 c. Each subgroup 204 a, 204 b . . . 204 i includes two output ports electrically coupled to a subgroup select circuit 206 a, 206 b . . . 206 i, each subgroup select circuit 206 a, 206 b . . . 206 i includes two output ports coupled electrically to a group select circuit 210 a, 210 b and 210 c associated with a particular group 202 a, 202 b, or 202 c. Each group select circuit 210 a, 210 b and 210 c includes two output ports coupled electrically to a common differential output bus 212 which terminates at a readout circuit 116.

The readout circuit 116 can amplify a signal it receives to facilitate further processing of the signal downstream of the readout circuit 116. The readout circuit 116 can require a considerable amount of power to accomplish these functions. The foregoing techniques can be used to help reduce the power requirements of the readout circuit 116.

Subgroup select circuits 206 a, 206 b . . . 206 i and group select circuits 210 a, 210 b, 210 c have associated values of parasitic capacitance values C_(par) that are approximately the same for each circuit. Also, the approximate value of parasitic capacitance for a given switch does not change significantly whether the switch is in a conductive or a non-conductive state.

Assuming that group select circuit 210 a is in a conductive state and group select circuits 210 b and 210 c are in a non-conductive state, the total parasitic capacitance that can be seen at the input terminals of the readout circuit 116 is approximately equal to the sum of the parasitic capacitances of circuits 206 a, 206 b, 206 c, 210 a, 210 b, and 210 c, or six times C_(par). If group select circuits were not included in the implementation illustrated in FIG. 4, the parasitic capacitance would be approximately equal to the sum of the parasitic capacitances of circuits 206 a, 206 . . . 206 i, or nine times C_(par). Therefore, integration of group select circuits 210 a, 210 b and 210 c can provide a reduction in the parasitic capacitance from nine times C_(par) to six times C_(par) at the input terminals of the readout circuit 116. With such a reduction of parasitic capacitance, a corresponding reduction in capacitive current losses also can be realized. Accordingly, the power required by the readout circuit 116 can be reduced as well.

As illustrated in the above example, the optimum number of group select circuits 210 a, 210 b, 210 c to minimize parasitic capacitance C_(par) should be approximately equal to the square root of the number of subgroup select circuits 206 a, 206 b . . . 206 i in the system. For example, if there are nine subgroup select circuits 206 a, 206 b . . . 206 i, then three group select circuits 210 a, 210 b and 210 c should be used. If additional levels of selection circuits are added downstream from the group select circuits 210 a, 210 b, 210 c, a similar design guideline may be applied. For each particular level of selection circuitry, the number of selection circuits to be implemented for reducing parasitic capacitance can be optimized. To do so, the number of selection circuits should be approximately equal to the square root of the number of selection circuits present in the layer immediately upstream from it.

Some array designs include 100 subgroups, 280 subgroups, 480 subgroups or 900 subgroups. Each subgroup would have a corresponding subgroup select circuit. Based on the foregoing guideline for optimizing the number of group select circuits, the 100 subgroup select circuits would be coupled to approximately 10 group select circuits, the 280 subgroup select circuits would be coupled to approximately 17 group select circuits, the 480 subgroup select circuits would be coupled to approximately 22 group select circuits and the 900 subgroup select circuits would be coupled to approximately 30 group select circuits. In these examples, the ratio of subgroup select circuits to group select circuits ranges from approximately 10:1 to 40:1 and preferably from approximately 15:1 to 30:1.

As shown in FIG. 5, the techniques can be applied to multiple levels of selection circuitry, implemented, for example, as a set of supergroup select circuits 500 a . . . 500 k. Each supergroup select circuit 500 a . . . 500 k is coupled to receive output signals from a set of group select circuits 210 a, 210 b . . . 210 y associated with a supergroup select circuit 500 a . . . 500 k. A supergroup can be considered to be a group of groups. The controller 104 can provide control signals at the gate terminals of each supergroup select circuit 500 a . . . 500 k to selectively enable each supergroup select circuits 500 a . . . 500 k to pass a signal, for example, from a supergroup input bus 502 to a common output bus 212. Each supergroup select circuit 500 a . . . 500 k can be implemented using a transistor switch with a respective gate terminal for receiving the control signal from the controller 104. The control signals can selectively enable each supergroup select circuit 500 a . . . 500 k in cooperation with the group select circuits 210 a, 210 b . . . 210 g and the subgroup select circuits 206 a, 206 b . . . 206 y to allow passage of signals from each subgroup select circuit 206 a, 206 b . . . 206 y to the readout circuit 116, one at a time, sequentially, similarly as outlined in detail above.

Various modifications to the foregoing techniques are possible. Other signal conditioning or signal sampling circuitry may be included according to the particular system requirements of each application. Additionally, the sequence of switching for the group select circuits 210 a, 210 b . . . 210 g and the subgroup select circuits 206 a, 206 b . . . 206 y may be varied so that the readout circuit 116 receives signals from different areas of the array 102 in a different order than the order described herein.

Other implementations are within the scope of the following claims. 

1. An apparatus comprising: groups of image sensors, each group comprising subgroups of sensors; subgroup select circuits, each of which is coupled to an output from a respective subgroup of sensors; group select circuits, each of which is coupled to outputs from subgroup select circuits associated with a respective one of the groups; a first bus for each group coupled to the outputs of the associated subgroup select circuits; a common output bus for receiving signals from each of the groups, the common output bus connected to the readout circuit; a controller for providing control signals to the subgroup select circuits and the group select circuits to selectively enable the respective subgroup select circuits and group select circuits to pass signals in a charge mode manner from the sensors to a readout circuit one sensor at a time; and a first isolation circuit coupled to each first bus for selectively isolating each group from the readout circuit, the first isolation circuit comprising a switch for selectively connecting the first bus to a predetermined voltage.
 2. The apparatus of claim 1 wherein the sensors comprise active pixel sensors.
 3. The apparatus of claim 1 further comprising a second bus coupled to the outputs of the group select circuits and the readout circuit.
 4. The apparatus of claim 1 wherein the number of group select circuits is approximately equal to the square root of the number of subgroup select circuits.
 5. The apparatus of claim 1 wherein a ratio of the number of subgroup select circuits to group select circuits is in a range of 10:1 and 40:1.
 6. The apparatus of claim 5 wherein the ratio is in a range of 15:1 and 30:1.
 7. The apparatus of claim 1 wherein each group select circuit comprises a transistor switch with a respective gate terminal for receiving a control signal from the controller, wherein when the switch is turned on, the group select circuit is enabled to pass signals from associated subgroup select circuits to the first bus, and when the switch is turned off, the group select circuit is disabled from passing signals from the associated subgroup select circuits to the first bus.
 8. The apparatus of claim 7 wherein each subgroup select circuit comprises a transistor switch with a respective gate terminal for receiving a control signal from the controller, wherein when the subgroup select circuit switch is turned on, the subgroup select circuit is enabled to pass signals from an associated subgroup of sensors, and when the subgroup select circuit switch is turned off, the subgroup select circuit is disabled from passing signals from the associated subgroup of sensors to the first bus.
 9. The apparatus of claim 8 wherein the controller is configured for generating the control signals to enable and disable the group select circuit switches and the subgroup select circuit switches in a predetermined sequence.
 10. The apparatus of claim 9 wherein the controller is configured to provide the control signals to enable the switches in the group select circuits sequentially, and, while a particular group select switch is enabled, to enable the subgroup select circuits associated with the particular group select circuit sequentially, one at a time.
 11. The apparatus of claim 1 wherein the controller is configured to enable and subsequently disable each group select circuit, one at a time, in a sequential manner.
 12. The apparatus of claim 1 wherein each group select circuit comprises a pair of NMOS transistor switches.
 13. The apparatus of claim 3 comprising: supergroups of sensors; supergroup select circuits, each of which is coupled to outputs from group select circuits associated with a respective one of the supergroups; and a second isolation circuit coupled to each second bus for selectively isolating each supergroup from the readout circuit, the second isolation circuit comprising a switch for selectively connecting the second bus to a predetermined voltage, wherein the controller is configured to provide control signals to the supergroup select circuits to selectively enable a supergroup select circuit to pass a signal from the second bus to a third, common output bus.
 14. The apparatus of claim 13 wherein an output of each supergroup select circuit is coupled electrically to a common output bus.
 15. The apparatus of claim 13 wherein each supergroup select circuit comprises a transistor switch with a respective gate terminal for receiving a control signal from the controller.
 16. The apparatus of claim 1, wherein the predetermined voltage is one of a supply voltage for the apparatus or a ground potential.
 17. The apparatus of claim 1, wherein the common output bus is connected between outputs from the group select circuits and the readout circuit.
 18. The apparatus of claim 13, wherein the common output bus is connected between outputs from the supergroup select circuits and the readout circuit.
 19. A method of obtaining a readout of a pixel sensor array comprising: organizing pixel sensors of an array into groups comprising a plurality of subgroups of sensors, the outputs of each subgroup coupled to an associated subgroup select circuit and the outputs of each subgroup select circuit coupled to an associated group select circuit; selectively enabling a group select circuit and a subgroup select circuit to allow a signal from each sensor in said subgroup to pass sequentially and in a charge mode manner to a readout circuit electrically connected to said group select circuit; sensing a charge at the readout circuit; and selectively isolating from said readout circuit the groups of pixels not associated with the selectively enabled group select circuit by disabling the subgroup select circuits and group select circuits not associated with the selectively enabled group select circuit and applying a predetermined voltage to at least one first bus coupled to the disabled group select circuits by operating a switch coupled to the at least one first bus.
 20. The method of claim 19 further comprising the step of organizing groups of subgroups of said sensors into supergroups, wherein the outputs of group select circuits associated with a supergroup are connected to a respective supergroup select circuit.
 21. The method of claim 20 further comprising the step of coupling each supergroup select circuit to an associated second bus, wherein the readout signals from each sensor in said supergroup will travel through said second bus.
 22. The method of claim 21 further comprising the step of selectively applying the predetermined voltage at each second bus.
 23. The method of claim 19, wherein the predetermined voltage is one of a supply voltage for the array or a ground potential. 